In welding robots for resistance welding, a welding head equipped with the required welding electrodes or welding tools is supported by an arm composed of two or more members connected with each other and with a stationary support member by adjustable pivot joints. The welding current may be supplied to the welding head by a flexible cable connecting the welding head to a stationary welding transformer. This system is, however, not satisfactory. After a period of operation, the stresses to which the cable is subjected by the repeated bending will cause fatigue breakage of the cable conductors to occur. As the cable has necessarily to be thick and heavy to carry the large currents required in resistance welding (which may be of the order of 10,000 amps. or more), the cable is liable to hamper the movements required for adjusting the position of the gun and also to impair the precision of the adjustment. For these reasons, it has already been suggested to replace the welding cable by a plurality of conductor elements connected in series by means of contact devices arranged at the pivot joints. Each of said contact devices consists of a pair of flat contact plates extending at right angles to the pivot axis and arranged in close juxtaposition. One of the contact plates is secured against rotation with respect to one of the members connected by the pivot joint, and the other contact plate is secured against rotation with respect to the other of said members. Each of the conductor elements is connected between two contact plates secured against rotation with respect to one and the same member. With this arrangement, adjustment of the pivot joints submits the conductor elements to no stresses or changes of shape whatever.
The system described is, however, not without serious disadvantages. In order to ensure a sufficiently low transition resistance, the contact plates have to be kept in engagement with each other by means exerting a strong mechanical pressure. Consequently, the motor and transmission means used for effecting the required angular adjustments of the members connected by the pivot joints have to be capable of providing a comparatively large torque in order to overcome the friction resulting from the contact pressure of the contact plates. This requirement is contrary to other desirable properties of the motor and transmission means, to wit, that it shall be capable of a swift adjusting action and that it shall occupy a minimum of space.